There has been known a snow depth sensor in which a projector projects a light ray in the form of a beam to a snow surface as a measurement target, and a photo detector in a photo receiver evaluates light reception of the light reflected from the snow surface to measure the depth of snow accumulation (see Japanese Patent Application Publication No. S58-87486). In the snow depth sensor in Japanese Patent Application Publication No. S58-87486, modulated light is projected and a photo detector element in the photo detector performs synchronous detection on the received light based on a modulation frequency. This processing extracts only a component of the projected light from the light reflected from the snow surface, and thereby reduces an influence of the ambient light.
In general, however, the photosensitivity characteristics of a photo detector element include a low sensitivity region on a high luminance side or a low luminance side. For this reason, if, for example, a sunny area and a shaded area are contained in a captured image, contrast in a synchronous detection image varies largely between these areas, and therefore it is not possible to obtain a synchronous detection image being stable in overall image contrast.